"For like grass they will soon wither. The Lord said, 'The wicked are nothing but grass; their hope is in vain.' As it says, 'When the wicked flourish like grass and all evildoers thrive, it is only so that they may be destroyed forever' (Psalm 92:8-9). Similarly, in Job it says, 'He is like a plant that grows in the sun, spreading its roots over a garden; it may grow beautifully, but when the scorching winds blow, it withers away' (Job 8:16-19). How long will they be like this, before they are consumed by fire? As it says, 'They will be trampled down like thorns, burning like fire' (Psalm 118:12). And that is why they will be harvested like weeds."...
The Wicked Wither Like Grass in the Sun
Midrash Tehillim 37:2